1. Field of the Invention
The present invention relates to an image processing apparatus operable in plural operation modes, and more particularly, to an image processing apparatus operable in plural operation modes having priority orders such as an electrophotographic copying machine comprising analogue and digital copying modes.
2. Description of Related Art
Conventionally, as electrophotographic copying machines, there have been provided an analogue copying machine for forming a latent image on a photoconductor and developing the latent image into a visible toner image so as to print an so-called analogue image on a sheet of copying paper, and a digital copying machine for printing dot images or so-called digital image on a sheet of copying paper according to digital image data.
In the digital copying machine, after an image of a document (referred to as a document image hereinafter) is read and converted into analogue electric signals by an image sensor, the analogue electric signals are converted into digital image data by an analogue to digital converter. Further, an exposure head such as an LED head forms dot images on a photoconductive drum according to the digital image data so as to form an electrostatic latent image thereon. Thereafter, in the manner well known to those skilled in the art, the electrostatic latent image is developed into a visible toner image with toner, and the toner image is transferred onto a sheet of copying paper.
The digital copying machine has such advantages that not only editing operation such as trimming operation can be performed since digital image data can be processed by a digital electric circuit but also digital image data can be stored in a storage unit and can be sent to an external unit.
On the other hand, the digital copying machine has such disadvantages that it is difficult not only to reproduce a half tone image but also to obtain a resolution higher than a predetermined resolution. In order to reproduce a half tone image, the dither matrix method is used generally. However, in this method, the resolution of the reproduced half tone image becomes low, resulting in degradation in the quality of the reproduced binary image such as character image or the like. Further, in order to heighten the resolution of the reproduced image, it is necessary to process a large amount of data for representing an image. Therefore, the digital copying machine requires not only a storage unit having a large memory capacity but also a longer processing time. Furthermore, when an image which has copied by a digital copying machine is recopied by a digital copying machine, in many cases, degradation in the quality of the recopied image is caused due to the moire phenomenon. Furthermore, when a half tone image which has copied by a digital copying machine is recopied by a digital copying machine, there is caused remarkable degradation in the quality of the recopied image which is produced by a so-called generation copying operation. Therefore, it is necessary to utilize either an analogue copying machine or a digital copying machine depending on the use for copying an image and/or a document.
In order to solve the aforementioned problems of the conventional digital copying machine, there are proposed a method for reproducing both of a binary image and a half tone image by processing image data so as to obtain a predetermined resolution, a method for performing the generation copying operation by reducing the moire phenomenon using a filtering method, and a method for reading an image with use of an image reader having a higher resolution and for writing the read image in a high density so as to reproduce an image in a higher quality on a sheet of copying paper. However, in all the methods, the composition of the copying machine becomes complicated, resulting in increase in the manufacturing cost and decrease in the processing speed thereof.
On the other hand, documents which are copied in offices are binary information such as characters, numerals, line drawings, and also it is necessary to often produce a hard copy of image data of an half tone image. Therefore, the analogue copying machine has higher performance in works in offices than the digital copying machine, generally.
In order to perform various kinds of copying operation, there has been proposed a copying machine of composite type comprising an analogue copying system and a digital copying system. In the copying machine of this type, the analogue and digital copying systems utilize a common electrophotographic printing section. However, the analogue copying system utilizes an exposure optical system for projecting light reflected from a document onto a photoconductor. On the other hand, the digital copying system utilizes an image read section using a CCD image sensor or the like, and an image write section such as laser printer or the like. Either the analogue copying system or the digital copying system of the copying machine of composite type can be used depending on the use so as to make the best use of the features of each copying system. For example, when a copy which has been produced by the digital copying system is recopied, degradation in the quality of the image can be reduced by reproducing the copy using the analogue copying system. Furthermore, an analogue half tone image and a digital binary image can be composed by the copying machine of composite type.
Generally, the copying machine of composite type comprises an image read mode for reading an image using a CCD image sensor which is used in a digital copying mode, and an image write mode for writing an image using an image write head such as an LED head, in addition to an analogue copying mode and the digital copying mode.
Respective processes of the analogue copying mode and the digital copying mode can be executed independent of the other apparatuses by the operator's operation. Further, in the process of the image read mode, the copying machine of this type is connected to an external unit such as a personal computer, and image data which have been read by the CCD image sensor are outputted thereto. In this case, the copying machine can be operated using an operation panel arranged on the external unit, however, it is necessary to arrange the external unit adjacently to the copying machine of composite type.
Furthermore, in the process of the image write mode, the copying machine of composite type may be connected to an external unit such as a personal computer, or the copying machine of composite type may be used as a facsimile by adding a telecommunication control section thereto. The latter operation mode is referred to as a facsimile mode hereinafter. In the facsimile mode, the copying machine of composite type is operable by a remote control through a telecommunication line.
Upon executing the process of the facsimile mode, when the process of either the analogue copying mode or the digital copying mode is in execution, image data sent from a transmitter can not be received. Therefore, not only the transmitter must wait without any operation but also the telecommunication line is occupied uselessly. Furthermore, processes of a personal computer which is connected to the transmitter are suspended, resulting in possibility of decrease in the working efficiency thereof.
In order to solve the aforementioned problems, there is provided a storage unit having a large memory capacity. In this case, image data sent from the transmitter are received and stored temporarily in the storage unit, and the image data can be printed on a sheet of copying paper after the copying operation is completed. However, the cost of the copying machine comprising the storage unit having a large memory capacity increases remarkably.
In the conventional copying machine of composite type, the process of the image read mode can be executed whether or not the process of the image write mode is in execution, however, the process of the image read mode can not be executed simultaneously when the process of either the analogue copying mode or the digital copying mode is in execution. Thus, among the processes of these modes of the copying machine of composite type, there are not only the process of a mode which should be executed by priority to the processes of the other modes but also the processes of the modes which can not be executed simultaneously.